Effects on N-Terminal L-Amino Acid Residues on Helical Screw Sence in Achiral Peptides

摘要

To understand the effects on N-terminal L-residues on dominating helical screw sense in achiral peptides, we adopted six kinds of peptides Box-X-(Aib-(#DELTA#)Phe)_2-Aib-OMe (Boc, t-butoxycarbonyl; OMe, methoxy), in which the X residue is an L-amino acid of alanine (Ala), leucine (Leu), valine (Val), phenylalanine (Phe), 1-naphthylalanine (Nap),or proline (Pro). The segment-(Aib-(#DELTA#)Phe)_2- was used for a backbone composed of two "enantiomeric" (left-/right-handed) helices. Actually, this could be confirmed by ~H MR and CD spectroscopy on Boc-(Aib-(#DELTA#)Phe)_2-Aib-OMe, which took left-and right-handed 3_(10)-helices with the same content. All peptides wee also found to take 3_(10)-type helical conformations in CDCl_3 from solvent accessibility of NH resonances. Chloroform, acetonitrile, methanol, and tetrahydrofuran were used for solvents in CD measurement. All peptides in all solvents showed marked exciton couplets around 280 nm with a positive peak at longer wavelengths. Consequently, when an N-terminal L-residue, irrespective of types of L-residues, is introduced into a helical segment of achiral peptide, its main chain prefers the left-handed screw sense. The peptide with X = Ala showed the smallest amplitude of exciton couplets in each solvent, meaning that the Ala residue with the smallest side chain (methyl group) had the least effective chirality for taking a one-side helical screw sense preferentially, compared with the other residues used here.